Search for Heavy Stable and Long-Lived Particles in $e^+ e^-$ Collisions at $\sqrt{s}$=189 GeV

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Search for Heavy Stable and Long-Lived Particles in

e+e− Collisions at √s=189 GeV

P. Abreu, W. Adam, T. Adye, P. Adzic, Z. Albrecht, T. Alderweireld, G.D. Alekseev, R. Alemany, T. Allmendinger, P.P. Allport, et al.

To cite this version:

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CERN{EP-2000-020

31January2000

Search for Heavy Stable and

Long-Lived Particles in e + e , Collisions at p s=189 GeV DELPHI Collaboration Abstract

A search for stable and long-lived heavy charged particles was performed using the data taken by the DELPHI experiment at an energy of 189 GeV. The Cherenkov light detected in the Ring Imaging Cherenkov Detector and the ionisation loss measured in the Time Projection Chamber were used to identify heavy particles passing through the detector. No evidence for the production of such particles has been found, therefore exclusion limits at 95% condence level were derived on the masses of left and right handed smuons and staus. The results were combined with previous DELPHI searches in this channel. Including previous DELPHI results, masses of left (right) handed stable smuons and staus can be excluded between 2 GeV/c2_{and 88 (87.5) GeV/c}2 _{at 95% CL.}

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P.Abreu22, W.Adam52, T.Adye38, P.Adzic12, Z.Albrecht18, T.Alderweireld2, G.D.Alekseev17, R.Alemany51,

T.Allmendinger18, P.P.Allport23, S.Almehed25, U.Amaldi9;29, N.Amapane47, S.Amato49, E.G.Anassontzis3,

P.Andersson46, A.Andreazza9, S.Andringa22, P.Antilogus26, W-D.Apel18, Y.Arnoud9, B.Asman46, J-E.Augustin26,

A.Augustinus9, P.Baillon9, P.Bambade20, F.Barao22, G.Barbiellini48, R.Barbier26, D.Y.Bardin17, G.Barker18,

A.Baroncelli40, M.Battaglia16, M.Baubillier24, K-H.Becks54, M.Begalli6, A.Behrmann54, P.Beilliere8, Yu.Belokopytov9,

K.Belous44, N.C.Benekos33, A.C.Benvenuti5, C.Berat15, M.Berggren24, D.Bertrand2, M.Besancon41, M.Bigi47,

M.S.Bilenky17, M-A.Bizouard20, D.Bloch10, H.M.Blom32, M.Bonesini29, M.Boonekamp41, P.S.L.Booth23,

A.W.Borgland4, G.Borisov20, C.Bosio43, O.Botner50, E.Boudinov32, B.Bouquet20, C.Bourdarios20, T.J.V.Bowcock23,

I.Boyko17, I.Bozovic12, M.Bozzo14, M.Bracko45, P.Branchini40, R.A.Brenner50, P.Bruckman9, J-M.Brunet8, L.Bugge34,

T.Buran34, B.Buschbeck52, P.Buschmann54, S.Cabrera51, M.Caccia28, M.Calvi29, T.Camporesi9, V.Canale39, F.Carena9,

L.Carroll23, C.Caso14, M.V.Castillo Gimenez51, A.Cattai9, F.R.Cavallo5, V.Chabaud9, M.Chapkin44, Ph.Charpentier9,

P.Checchia37, G.A.Chelkov17, R.Chierici47, P.Chliapnikov9;44, P.Chochula7, V.Chorowicz26, J.Chudoba31, K.Cieslik19,

P.Collins9, R.Contri14, E.Cortina51, G.Cosme20, F.Cossutti9, H.B.Crawley1, D.Crennell38, S.Crepe15, G.Crosetti14,

J.Cuevas Maestro35, S.Czellar16, M.Davenport9, W.Da Silva24, G.Della Ricca48, P.Delpierre27, N.Demaria9,

A.De Angelis48, W.De Boer18, C.De Clercq2, B.De Lotto48, A.De Min37, L.De Paula49, H.Dijkstra9, L.Di Ciaccio9;39,

J.Dolbeau8, K.Doroba53, M.Dracos10, J.Drees54, M.Dris33, A.Duperrin26, J-D.Durand9, G.Eigen4, T.Ekelof50,

G.Ekspong46, M.Ellert50, M.Elsing9, J-P.Engel10, M.Espirito Santo9, G.Fanourakis12, D.Fassouliotis12, J.Fayot24,

M.Feindt18, A.Ferrer51, E.Ferrer-Ribas20, F.Ferro14, S.Fichet24, A.Firestone1, U.Flagmeyer54, H.Foeth9, E.Fokitis33,

F.Fontanelli14, B.Franek38, A.G.Frodesen4, R.Fruhwirth52, F.Fulda-Quenzer20, J.Fuster51, A.Galloni23, D.Gamba47,

S.Gamblin20, M.Gandelman49, C.Garcia51, C.Gaspar9, M.Gaspar49, U.Gasparini37, Ph.Gavillet9, E.N.Gazis33, D.Gele10,

T.Geralis12, N.Ghodbane26, I.Gil51, F.Glege54, R.Gokieli9;53, B.Golob9;45, G.Gomez-Ceballos42, P.Goncalves22,

I.Gonzalez Caballero42, G.Gopal38, L.Gorn1, Yu.Gouz44, V.Gracco14, J.Grahl1, E.Graziani40, P.Gris41, G.Grosdidier20,

K.Grzelak53, J.Guy38, C.Haag18, F.Hahn9, S.Hahn54, S.Haider9, A.Hallgren50, K.Hamacher54, J.Hansen34, F.J.Harris36,

V.Hedberg9;25, S.Heising18, J.J.Hernandez51, P.Herquet2, H.Herr9, T.L.Hessing36, J.-M.Heuser54, E.Higon51,

S-O.Holmgren46, P.J.Holt36, S.Hoorelbeke2, M.Houlden23, J.Hrubec52, M.Huber18, K.Huet2, G.J.Hughes23,

K.Hultqvist9;46, J.N.Jackson23, R.Jacobsson9, P.Jalocha19, R.Janik7, Ch.Jarlskog25, G.Jarlskog25, P.Jarry41,

B.Jean-Marie20, D.Jeans36, E.K.Johansson46, P.Jonsson26, C.Joram9, P.Juillot10, L.Jungermann18, F.Kapusta24,

K.Karafasoulis12, S.Katsanevas26, E.C.Katsous33, R.Keranen18, G.Kernel45, B.P.Kersevan45, Yu.Khokhlov44,

B.A.Khomenko17, N.N.Khovanski17, A.Kiiskinen16, B.King23, A.Kinvig23, N.J.Kjaer9, O.Klapp54, H.Klein9, P.Kluit32,

P.Kokkinias12, V.Kostioukhine44, C.Kourkoumelis3, O.Kouznetsov17, M.Krammer52, E.Kriznic45, Z.Krumstein17,

P.Kubinec7, J.Kurowska53, K.Kurvinen16, J.W.Lamsa1, D.W.Lane1, V.Lapin44, J-P.Laugier41, R.Lauhakangas16,

G.Leder52, F.Ledroit15, V.Lefebure2, L.Leinonen46, A.Leisos12, R.Leitner31, G.Lenzen54, V.Lepeltier20, T.Lesiak19,

M.Lethuillier41, J.Libby36, W.Liebig54, D.Liko9, A.Lipniacka9;46, I.Lippi37, B.Loerstad25, J.G.Loken36, J.H.Lopes49,

J.M.Lopez42, R.Lopez-Fernandez15, D.Loukas12, P.Lutz41, L.Lyons36, J.MacNaughton52, J.R.Mahon6, A.Maio22,

A.Malek54, T.G.M.Malmgren46, S.Maltezos33, V.Malychev17, F.Mandl52, J.Marco42, R.Marco42, B.Marechal49,

M.Margoni37, J-C.Marin9, C.Mariotti9, A.Markou12, C.Martinez-Rivero20, F.Martinez-Vidal51, S.Marti i Garcia9,

J.Masik13, N.Mastroyiannopoulos12, F.Matorras42, C.Matteuzzi29, G.Matthiae39, F.Mazzucato37, M.Mazzucato37,

M.Mc Cubbin23, R.Mc Kay1, R.Mc Nulty23, G.Mc Pherson23, C.Meroni28, W.T.Meyer1, A.Miagkov44, E.Migliore9,

L.Mirabito26, W.A.Mitaro52, U.Mjoernmark25, T.Moa46, M.Moch18, R.Moeller30, K.Moenig9;11, M.R.Monge14,

D.Moraes49, X.Moreau24, P.Morettini14, G.Morton36, U.Mueller54, K.Muenich54, M.Mulders32, C.Mulet-Marquis15,

R.Muresan25, W.J.Murray38, B.Muryn19, G.Myatt36, T.Myklebust34, F.Naraghi15, M.Nassiakou12, F.L.Navarria5,

S.Navas51, K.Nawrocki53, P.Negri29, N.Neufeld9, R.Nicolaidou41, B.S.Nielsen30, P.Niezurawski53, M.Nikolenko10;17,

V.Nomokonov16, A.Nygren25, V.Obraztsov44, A.G.Olshevski17, A.Onofre22, R.Orava16, G.Orazi10, K.Osterberg16,

A.Ouraou41, M.Paganoni29, S.Paiano5, R.Pain24, R.Paiva22, J.Palacios36, H.Palka19, Th.D.Papadopoulou9;33, L.Pape9,

C.Parkes9, F.Parodi14, U.Parzefall23, A.Passeri40, O.Passon54, T.Pavel25, M.Pegoraro37, L.Peralta22, M.Pernicka52,

A.Perrotta5, C.Petridou48, A.Petrolini14, H.T.Phillips38, F.Pierre41, M.Pimenta22, E.Piotto28, T.Podobnik45, M.E.Pol6,

G.Polok19, P.Poropat48, V.Pozdniakov17, P.Privitera39, N.Pukhaeva17, A.Pullia29, D.Radojicic36, S.Ragazzi29,

H.Rahmani33, J.Rames13, P.N.Rato21, A.L.Read34, P.Rebecchi9, N.G.Redaelli29, M.Regler52, J.Rehn18, D.Reid32,

R.Reinhardt54, P.B.Renton36, L.K.Resvanis3, F.Richard20, J.Ridky13, G.Rinaudo47, I.Ripp-Baudot10, O.Rohne34,

A.Romero47, P.Ronchese37, E.I.Rosenberg1, P.Rosinsky7, P.Roudeau20, T.Rovelli5, Ch.Royon41, V.Ruhlmann-Kleider41,

A.Ruiz42, H.Saarikko16, Y.Sacquin41, A.Sadovsky17, G.Sajot15, J.Salt51, D.Sampsonidis12, M.Sannino14,

Ph.Schwemling24, B.Schwering54, U.Schwickerath18, F.Scuri48, P.Seager21, Y.Sedykh17, A.M.Segar36, N.Seibert18,

R.Sekulin38, R.C.Shellard6, M.Siebel54, L.Simard41, F.Simonetto37, A.N.Sisakian17, G.Smadja26, O.Smirnova25,

G.R.Smith38, O.Solovianov44, A.Sopczak18, R.Sosnowski53, T.Spassov22, E.Spiriti40, S.Squarcia14, C.Stanescu40,

S.Stanic45, M.Stanitzki18, K.Stevenson36, A.Stocchi20, J.Strauss52, R.Strub10, B.Stugu4, M.Szczekowski53,

M.Szeptycka53, T.Tabarelli29, A.Taard23, F.Tegenfeldt50, F.Terranova29, J.Thomas36, J.Timmermans32, N.Tinti5,

L.G.Tkatchev17, M.Tobin23, S.Todorova9, A.Tomaradze2, B.Tome22, A.Tonazzo9, L.Tortora40, P.Tortosa51,

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S.Tzamarias12, O.Ullaland9, V.Uvarov44, G.Valenti9;5, E.Vallazza48, P.Van Dam32, W.Van den Boeck2,

W.K.Van Doninck2, J.Van Eldik9;32, A.Van Lysebetten2, N.van Remortel2, I.Van Vulpen32, G.Vegni28, L.Ventura37,

W.Venus38;9, F.Verbeure2, P.Verdier26, M.Verlato37, L.S.Vertogradov17, V.Verzi28, D.Vilanova41, L.Vitale48, E.Vlasov44,

A.S.Vodopyanov17, G.Voulgaris3, V.Vrba13, H.Wahlen54, C.Walck46, A.J.Washbrook23, C.Weiser9, D.Wicke54,

J.H.Wickens2, G.R.Wilkinson36, M.Winter10, M.Witek19, G.Wolf9, J.Yi1, O.Yushchenko44, A.Zalewska19, P.Zalewski53,

D.Zavrtanik45, E.Zevgolatakos12, N.I.Zimin17;25, A.Zintchenko17, Ph.Zoller10, G.C.Zucchelli46, G.Zumerle37

1Department of Physics and Astronomy, Iowa State University, Ames IA 50011-3160, USA

2Physics Department, Univ. Instelling Antwerpen, Universiteitsplein 1, B-2610 Antwerpen, Belgium

and IIHE, ULB-VUB, Pleinlaan 2, B-1050 Brussels, Belgium

and Faculte des Sciences, Univ. de l'Etat Mons, Av. Maistriau 19, B-7000 Mons, Belgium

3Physics Laboratory, University of Athens, Solonos Str. 104, GR-10680 Athens, Greece 4Department of Physics, University of Bergen, Allegaten 55, NO-5007 Bergen, Norway

5Dipartimento di Fisica, Universita di Bologna and INFN, Via Irnerio 46, IT-40126 Bologna, Italy 6Centro Brasileiro de Pesquisas Fsicas, rua Xavier Sigaud 150, BR-22290 Rio de Janeiro, Brazil

and Depto. de Fsica, Pont. Univ. Catolica, C.P. 38071 BR-22453 Rio de Janeiro, Brazil

and Inst. de Fsica, Univ. Estadual do Rio de Janeiro, rua S~ao Francisco Xavier 524, Rio de Janeiro, Brazil

7Comenius University, Faculty of Mathematics and Physics, Mlynska Dolina, SK-84215 Bratislava, Slovakia 8College de France, Lab. de Physique Corpusculaire, IN2P3-CNRS, FR-75231 Paris Cedex 05, France 9CERN, CH-1211 Geneva 23, Switzerland

10Institut de Recherches Subatomiques, IN2P3 - CNRS/ULP - BP20, FR-67037 Strasbourg Cedex, France 11Now at DESY-Zeuthen, Platanenallee 6, D-15735 Zeuthen, Germany

12Institute of Nuclear Physics, N.C.S.R. Demokritos, P.O. Box 60228, GR-15310 Athens, Greece

13FZU, Inst. of Phys. of the C.A.S. High Energy Physics Division, Na Slovance 2, CZ-180 40, Praha 8, Czech Republic 14Dipartimento di Fisica, Universita di Genova and INFN, Via Dodecaneso 33, IT-16146 Genova, Italy

15Institut des Sciences Nucleaires, IN2P3-CNRS, Universite de Grenoble 1, FR-38026 Grenoble Cedex, France 16Helsinki Institute of Physics, HIP, P.O. Box 9, FI-00014 Helsinki, Finland

17Joint Institute for Nuclear Research, Dubna, Head Post Oce, P.O. Box 79, RU-101 000 Moscow, Russian Federation 18Institut fur Experimentelle Kernphysik, Universitat Karlsruhe, Postfach 6980, DE-76128 Karlsruhe, Germany 19Institute of Nuclear Physics and University of Mining and Metalurgy, Ul. Kawiory 26a, PL-30055 Krakow, Poland 20Universite de Paris-Sud, Lab. de l'Accelerateur Lineaire, IN2P3-CNRS, B^at. 200, FR-91405 Orsay Cedex, France 21School of Physics and Chemistry, University of Lancaster, Lancaster LA1 4YB, UK

22LIP, IST, FCUL - Av. Elias Garcia, 14-1o, PT-1000 Lisboa Codex, Portugal

23Department of Physics, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK

24LPNHE, IN2P3-CNRS, Univ. Paris VI et VII, Tour 33 (RdC), 4 place Jussieu, FR-75252 Paris Cedex 05, France 25Department of Physics, University of Lund, Solvegatan 14, SE-223 63 Lund, Sweden

26Universite Claude Bernard de Lyon, IPNL, IN2P3-CNRS, FR-69622 Villeurbanne Cedex, France 27Univ. d'Aix - Marseille II - CPP, IN2P3-CNRS, FR-13288 Marseille Cedex 09, France

28Dipartimento di Fisica, Universita di Milano and INFN-MILANO, Via Celoria 16, IT-20133 Milan, Italy

29Dipartimento di Fisica, Univ. di Milano-Bicocca and INFN-MILANO, Piazza delle Scienze 2, IT-20126 Milan, Italy 30Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen , Denmark

31IPNP of MFF, Charles Univ., Areal MFF, V Holesovickach 2, CZ-180 00, Praha 8, Czech Republic 32NIKHEF, Postbus 41882, NL-1009 DB Amsterdam, The Netherlands

33National Technical University, Physics Department, Zografou Campus, GR-15773 Athens, Greece 34Physics Department, University of Oslo, Blindern, NO-1000 Oslo 3, Norway

35Dpto. Fisica, Univ. Oviedo, Avda. Calvo Sotelo s/n, ES-33007 Oviedo, Spain 36Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

37Dipartimento di Fisica, Universita di Padova and INFN, Via Marzolo 8, IT-35131 Padua, Italy 38Rutherford Appleton Laboratory, Chilton, Didcot OX11 OQX, UK

39Dipartimento di Fisica, Universita di Roma II and INFN, Tor Vergata, IT-00173 Rome, Italy

40Dipartimento di Fisica, Universita di Roma III and INFN, Via della Vasca Navale 84, IT-00146 Rome, Italy 41DAPNIA/Service de Physique des Particules, CEA-Saclay, FR-91191 Gif-sur-Yvette Cedex, France 42Instituto de Fisica de Cantabria (CSIC-UC), Avda. los Castros s/n, ES-39006 Santander, Spain

43Dipartimento di Fisica, Universita degli Studi di Roma La Sapienza, Piazzale Aldo Moro 2, IT-00185 Rome, Italy 44Inst. for High Energy Physics, Serpukov P.O. Box 35, Protvino, (Moscow Region), Russian Federation

45J. Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia and Laboratory for Astroparticle Physics,

Nova Gorica Polytechnic, Kostanjeviska 16a, SI-5000 Nova Gorica, Slovenia, and Department of Physics, University of Ljubljana, SI-1000 Ljubljana, Slovenia

46Fysikum, Stockholm University, Box 6730, SE-113 85 Stockholm, Sweden

47Dipartimento di Fisica Sperimentale, Universita di Torino and INFN, Via P. Giuria 1, IT-10125 Turin, Italy 48Dipartimento di Fisica, Universita di Trieste and INFN, Via A. Valerio 2, IT-34127 Trieste, Italy

and Istituto di Fisica, Universita di Udine, IT-33100 Udine, Italy

49Univ. Federal do Rio de Janeiro, C.P. 68528 Cidade Univ., Ilha do Fund~ao BR-21945-970 Rio de Janeiro, Brazil 50Department of Radiation Sciences, University of Uppsala, P.O. Box 535, SE-751 21 Uppsala, Sweden

51IFIC, Valencia-CSIC, and D.F.A.M.N., U. de Valencia, Avda. Dr. Moliner 50, ES-46100 Burjassot (Valencia), Spain 52Institut fur Hochenergiephysik, Osterr. Akad. d. Wissensch., Nikolsdorfergasse 18, AT-1050 Vienna, Austria 53Inst. Nuclear Studies and University of Warsaw, Ul. Hoza 69, PL-00681 Warsaw, Poland

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1

1 Introduction

The search for heavy stable and long-lived charged particles in events with low mul-tiplicity was made within the data recorded by the DELPHI detector at LEP at center-of-mass energies of about 189 GeV. The results presented in this paper extend previous DELPHI results from [1] in the leptonic topology to energies of 189 GeV. In most Super-symmetricmodels (SUSY) the supersymmetricpartners of standard particles are unstable and have short lifetimes, except the lightest supersymmetric particle (LSP) which is com-monly believed to be neutral and stable. Therefore, in most of the searches it is assumed that the supersymmetric particles decay instantaneously. In some scenarious, however, it is possible that SUSY-particles acquire a long lifetime. They become long-lived or stable, and thus escape from detection by the standard searches. In the Minimal Supersymmet-ric Standard Model (MSSM) with a very small amount of R-parity violation the LSP can be a charged slepton or squark and decay with a long lifetime into Standard Model particles [2].

In gauge mediated supersymmetric models [3] the gravitino is the LSP and the next to lightest supersymmetric particle (NLSP) can obtain a long lifetime in a very natural way for large values of the SUSY-breaking scale [4]. This is possible for sleptons, for example when the stau is the NLSP. The pair production of such long-lived or stable particles yields a characteristic signature, with typically two back-to-back charged heavy objects in the detector.

In this analysis it was assumed that the long-lived particles decay outside the tracking volume of the detector, which extends to a typical radius of 1.5 m. Furthermore, it was assumed that they do not interact more strongly than ordinary matter particles.

Heavy stable particles were searched for by looking for high momentum charged par-ticles with either anomalous ionisation loss (dE/dx) measured in the Time Projection Chamber (TPC), or the absence of Cherenkov light in the gas and liquid radiators of the Barrel Ring Imaging CHerenkov (RICH). The combination of the TPC and RICH detectors and kinematical cuts provided ecient detection of new heavy particles with a small background.

The data analysed corresponded to 155.3 pb,1 at an energy of about 189 GeV. The

negative search result was used to set limits at 95% CL on the production cross-section of stable sleptons. They were combined with previous DELPHI results from [1], and mass limits on stable or long-lived smuons and staus were derived.

2 Event selection

A description of the DELPHI apparatus and its performance can be found in ref. [5], with more details on the Barrel RICH in ref. [6]. Charged particles were selected if their impact parameter was less than 5 cm in the azimuthal plane and less than 10 cm in the longitudinal direction, and their polar angles lay between 20 and 160. The relative

errors on the measured momentum1 _{were required to be less than 100% and the track}

lengths larger than 30 cm.

Only events with two or three charged particles were considered. It was further re-quired that at least one charged particle had a momentum above 5 GeV/c reconstructed by the TPC and was inside the acceptance of the Barrel RICH jcos j<0:68, where is

the polar angle.

1Within this paper, with "momentum" the apparent momentum is meant, dened as the ratio of the momentum and

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Cosmic muons were removed by putting tighter cuts on the impact parameter with respect to the average beam-spot position. When the event had two charged particles with at least one identied muon in the muon chambers, the impact parameter in the azimuthal plane was required to be less than 0.15 cm, and less than 1.5 cm in the longitudinal plane. Charged particles were selected, if they fulllled a combination of the following criteria: 1. the Gas Veto: no photons were observed in the Gas RICH

2. the Liquid Veto: four or less photons were observed in the Liquid RICH

3. the TPC high ionisation loss: measured normalised energy loss was above 2 units i.e. twice the energy loss for a minimum ionising particle

4. the TPC low ionisation loss: measured normalised energy loss was below 0.3 of that expected for protons

The particle identication using the RICH is described in detail in ref. [7].

For the Gas and Liquid Vetoes it was required that the RICH was fully operational, and that photons from other tracks or ionisation hits were detected inside the drift tube crossed by the track. Due to tracking problems electrons often passed a Gas or Liquid Veto. Therefore it was required that particles that deposit more than 5 GeV in the electromagnetic calorimeter, had hits in the outer tracking detector. Signals from at least 80 wires were required for the measurement of the normalised energy loss in the TPC.

An event was selected if the momentum of a charged particle was above 15 GeV/c and the Gas Veto (1) was conrmed by a Liquid Veto (2) or a low ionisation loss (4) (in boolean notation (1)(2)+(1)(4)) or if the momentum of a charged particle was above 5

GeV/c and the Gas Veto was conrmed by a high ionisation loss ((1)(3)). The event was

also accepted if two charged particles were present with both momenta above 15 GeV/c, and both had either a high ionisation loss or a Gas Veto, or both had a low ionisation loss ((1+3)(1'+3')+(4)(4')). This additional search window improves the eciency for

large masses by about 10%.

3 Analysis results

One event was selected in the data, in agreement with the expected background of 1.02

0.13 events. The expected background was evaluated from the data by estimating the

probability of accepted particles passing the individual cuts which build up the dierent search windows. The largest background contribution was expected in the high energy loss search window, 0.540.1. A background of 0.340:07 was expected in the combined

Liquid and Gas Veto search window, whereas the contribution from the other two windows was small. The candidate event passed both the Gas and Liquid Vetoes. It had two high momentum back-to-back charged particles with associated hits in the muon chambers. Figure 1 shows the measured normalised ionisation loss and the measured Cherenkov angle in the liquid radiator for the data taken at 189 GeV, after applying the Gas Veto. The eciency for selecting an event was evaluated for right handed smuons as a func-tion of the mass in 10 GeV/c2 _{steps between 10 GeV/c}2 _{and 90 GeV/c}2_{. Additional}

points at 85,91,92 and 93 GeV/c2 _{have been added in order to get a better modelling of}

the eciency close to the kinematical limit. At each point 1000 events were simulated with SUSYGEN [8], and passed through the detector simulation. The eciency curve for masses above 45 GeV/c2 _{and a centre-of-mass energy of 189 GeV is shown in Figure}

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3

As no evidence for a signal was found, exclusion limits at 95% CL were derived. The new results were combined with previous published results from [1] with a likelihood ratio method to obtain experimental limits on the masses of left and right handed stable or long-lived smuons and staus. For selectrons, no limitswere given because in this model the production cross-section can be highly suppressed due to an addition t-channel sneutrino exchange. The candidate event has been taken into account over the full mass range. Figure 2b shows the cross-section for left and right handed smuons in the MSSM at an energy of 189 GeV, calculated with SUSYGEN [8] as a function of the particle mass. The 95% CL cross-section limit is also shown. The production of left (right) handed stable smuons and staus can be excluded with masses up to 88 (87.5) GeV/c2_.

4 Conclusions

A search was made for stable and long-lived heavy charged particles in leptonic nal states at energies of 189 GeV, using particles identied by the Cherenkov light in the RICH and the ionisation loss in the TPC.

One event was observed in the data, in agreement with the expectation of 1.02

0.13 background events. The results were combined with previous results of the DELPHI Collaboration[1], which excluded long lived or stable left (right) handed smuons and staus for masses between 2 and 81 (80) GeV/c2_{. Including the new data, left (right)}

handed stable smuons and staus can be excluded with masses between 2 GeV/c2 _{and 88}

(87.5) GeV/c2 _{at 95% CL.}

Acknowledgements

We are greatly indebted to our technical collaborators, to the members of the CERN-SL Division for the excellent performance of the LEP collider, and to the funding agencies for their support in building and operating the DELPHI detector.

We acknowledge in particular the support of

Austrian Federal Ministry of Science and Tracs, GZ 616.364/2-III/2a/98, FNRS{FWO, Belgium,

FINEP, CNPq, CAPES, FUJB and FAPERJ, Brazil,

Czech Ministry of Industry and Trade, GA CR 202/96/0450 and GA AVCR A1010521, Danish Natural Research Council,

Commission of the European Communities (DG XII), Direction des Sciences de la Matiere, CEA, France,

Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie, Germany, General Secretariat for Research and Technology, Greece,

National Science Foundation (NWO) and Foundation for Research on Matter (FOM), The Netherlands,

Norwegian Research Council,

State Committee for Scientic Research, Poland, 2P03B06015, 2P03B1116 and SPUB/P03/178/98,

JNICT{Junta Nacional de Investigac~ao Cientca e Tecnologica, Portugal, Vedecka grantova agentura MS SR, Slovakia, Nr. 95/5195/134,

Ministry of Science and Technology of the Republic of Slovenia, CICYT, Spain, AEN96{1661 and AEN96-1681,

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Particle Physics and Astronomy Research Council, UK, Department of Energy, USA, DE{FG02{94ER40817.

References

[1] P. Abreu et al. (DELPHI Collaboration), Phys. Lett.B444 (1998) 491

[2] H. Dreiner, hep-ph/9707435.

[3] G.F. Giudice and R. Rattazzi, hep-ph/9801271. [4] M. Dine et al., Phys. Rev. Lett.76 (1996) 3484;

S. Dimopoulos et al., Nucl. Phys. B Proc. Suppl. 52A (1997) 38.

[5] P. Aarnio et al.(DELPHI Collaboration), Nucl. Instr. Meth.A 303 (1991) 233;

P. Abreu et al.(DELPHI Collaboration), Nucl. Instr. Meth.A378 (1996) 57.

[6] J. Seguinot and T.Ypsilantis, Nucl. Instr. Meth.A343 (1994) 1;

W. Adam et al., Nucl. Instr. Meth.A343 (1994) 68.

[7] W. Adam et al., Nucl. Instr. Meth.A371 (1996) 240.

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5

DELPHI slepton searches at 189 GeV

0 1 2 3 4 5 6 7 8 9 10 5.0 15. 45. 135. Apparent momentum p/|q| (GeV/c)

normalised dE/dx q=±e q=±2/3 e 90 GeV/c2 Signal MC 0 0.2 0.4 0.6 0.8 1 5.0 15. 45. 135. Apparent momentum p/|q| (GeV/c)

Cherenkov angle liquid (rad)

Figure 1: (a) Normalised energy loss as a function of the apparent momentum p/jqj

after the Gas Veto for the 189 GeV data. (b) Measured Cherenkov angle in the liquid radiator as a function of the apparent momentum after the Gas Veto: if four photons or less were observed in the liquid radiator, the Cherenkov angle was set equal to zero. The rectangular areas in (a) indicate selections (1)(2) and (1)(4), and that in (b) shows

selection (1)(3). The selection criteria are explained in the text. Open circles are data.

The small lled circles indicate the expectation for a 90 GeV/c2 _{signal with charge} e,

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 45 50 55 60 65 70 75 80 85 90 m_µ(GeV/c 2) Efficiency s=189 GeV 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 70 72.5 75 77.5 80 82.5 85 87.5 90 92.5 m_µ(GeV/c 2) σ (e + e - -> µ + µ - )(pb) 95% CL s=133-189 GeV right handed left handed ~ ~ ~~

DELPHI

a) b)

Figure 2: (a) Eciency for detecting stable and long-lived smuons (staus) as a function of the smuon mass at a centre-of-mass energy of 189 GeV.